Alright, let me give it a go:
USA people:.
Who has a more "posh" accent? Mick Jagger or Paul McCartney?

If you're a Fawlty Towers fan, is Sybil from the north or south of England? And is Sybil's or Bails perceived as more posh?
.I'm asking this because I see people say that all british ascents sound the same, and I don't hear that at all.

Some american phrases and speech patterns sound old fashioned or even archaic to me, but damned if I can think of one now. "Oftentimes", maybe.

Anyway, OP's awfully confident of that for someone without a time machine. I'm sure Brits have experienced drift in terms of accent but I'd be astonished if we'd changed but American accents had stayed static.

Seems like we had a thread about some British singers sounding American (when singing). I read a biography of Led Zeppelin where they noted that a lot of teenagers in America in the 70's thought the band was American. I can include myself in that group. Perhaps it had to do with the influence of old blues and other American music on British groups and they were imitating those vocal styles.

I remember being surprised, as a child, to hear a pop star interviewed after performing a song and learn that he did not have a British accent. It was the mid-1960s and I assumed anyone who had a song on the radio was British, regardless of what they sounded like when they sang.

Both British and America accents would have changed in over 200 years but retained some older usages. A good example is the British still say "darby" for derby, "barclay" for Berkeley but both the Americans and British still pronounce sergeant as "sargent". "er" sounds were often rendered as "ar" in the 18th Century- "marchant", "parson", "marcy" for merchant, person and mercy. Tea and sea were usually pronounced "tay" and "say" by most folk in the 18th Century. Some of those pronunciations have been retained in what we think of a hillbilly accents. The flat "A" has been mostly lost in many words such a father which would have been said with an "a" as in bat. Geordies in the UK still sometimes pronounce father that way even now and use the flat "a" in bath, grass, etc.
Considering some of the diaects in the UK would have been preserved by their remoteness or the limited communities in which they existed- mining villages etc I can't see them as ever resembling the modern day American accent anytime in the past.
Both US and British English have evolved since the Revolution sometimes seperately but now due to cinema TV and the internet they're swopping expressions quite a lot- no Englishman would have said "******" even 10 years ago, and Americans wouldn't have used "shag" or "bollocks" but some certainly do now.